This invention relates to novel means for stabilizing the xylene isomerization activity of a dual bed xylene isomerization catalyst system. This invention is useful for isomerizing a feed containing an aromatic C8 mixture of xylenes and ethylbenzene (EB) in which the para-xylene content of the xylene-containing portion of the feed is less than the equilibrium content, to produce a product stream of reduced ethylbenzene content and a greater amount of desired para xylene. Para-xylene is an important hydrocarbon feed material for the manufacture of terephthalic acid. The present invention comprises placing a hydrogenation catalyst bed, for example, a molybdenum-on-alumina (Mo/alumina) catalyst bed, between ethylbenzene conversion and xylene isomerization catalyst components to provide an improved catalyst system in which deactivation of the xylene isomerization catalyst component occurs less rapidly.
As commercial use of xylenes has increased, there has been continuing interest in methods to isomerize the xylene isomers obtained from processing aromatic naphthas toward an equilibrium mix to increase yields of the more desirable para-xylene. Para-xylene is useful in the manufacture of terephthalic acid which is an intermediate in the manufacture of polyesters. Typically para-xylene is derived from mixtures of C8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by distillation. The C8 aromatics in such mixtures are ethylbenzene, para-xylene, meta-xylene, and ortho-xylene.
A dual bed catalyst system for converting ethylbenzene and non-aromatics in mixed ethylbenzene xylene containing feeds, while simultaneously converting xylenes to thermal equilibrium is disclosed in U.S. Pat. No. 4,899,011.
A dual bed xylene isomerization catalyst consists of an EB conversion catalyst component and a xylene isomerization component. Typically, the EB conversion catalyst is selective for converting EB to products which can be separated via distillation, while being an ineffective xylene isomerization catalyst, that is, it does not produce an equilibrium distribution of xylene isomers. Such a catalyst system is described in U.S. Pat. No. Re 31,782. This catalyst system has an advantage over a conventional single bed xylene isomerization catalyst in that it affords lower xylene losses. However, catalysts such as those described in U.S. Pat. No. Re 31,782 give unacceptably high deactivation rates. The rapid deactivation of the xylene isomerization catalyst component is believed to be caused by the ethylene that is generated over the EB conversion catalyst. In experiments using only the EB conversion catalyst, we found that the C2 component in the gaseous component of the product consists of more than 80% ethylene.
We solved this deactivation problem by placing a hydrogenation catalyst bed between the EB conversion catalyst component and the xylene isomerization catalyst, referred to herein as a xe2x80x9csandwichxe2x80x9d bed. In an experiment with the EB conversion catalyst and a hydrogenation catalyst bed in the form of a Mo/alumina catalyst bed below it, we demonstrated that the ethylene produced over the EB conversion catalyst was quantitatively hydrogenated to ethane without any detrimental effect to the other components in the product.